A Theoretical Study of Time Delays in Abovethreshold ω-2ω Ionization

S NAGELE; DONSA, S.; ARBÓ, D. G.; LÓPEZ, S D; J. BURGDÖRFER

Paris

Simposio; The 5th International Symposium on Intense Short Wavelenght Processes in Atoms and Molecules ISWAMP; 2019

An experiment in atomic ionization by two-color lasers by Zipp et al.1 has revealed that apump-probe scheme can be used to characterize time delays in the emission of electrons inthe above-threshold ionization regime for visible frequency of the pump and its first harmonicas a probe. To shed more light about the ionization processes of the mentioned experiment,in this work we perform a theoretical analysis of the time delays in Ar ionization by a twocolor laser [Ti:Sapphire laser (800 nm) together with the first harmonic (400 nm)] for a typicalω−2ω configuration 2. We perform simulations with the time dependent Schödinger equation3and compare these results with the strong-field and Coulomb-Volkov approximations. Wefind that the strong assumption of additive time delays adopted in streaking or RABBITTtechniques needs to be revisited when the atomic system is subject to ω−2ω fields 4. Weshow that time delays depend on the definition used to extract them from electronmomentum distributions by considering the asymmetry in a particular direction of the electronemission or by integrating in hemispheres, and also considering a single energy at themultiphoton peaks or integrating over energy around the multiphoton peaks. Besides, wealso find a large discrepancy between the results predicted by the strong-field approximationand the numerical solutions of the time dependent Schrödinger equation even at the highestsimulated energies. With the help of the Coulomb-Volkov approximation, we see that thecontribution of the long-range potential effects is almost negligible at intermediate to highemission energies with respect to the strong field approximation. We have also comparedwith ab initio simulations using short-range Yukawa potentials.REFERENCES1. L. J. Zipp, A. Natan, and P. H. Bucksbaum, Optica 1, 361-364 (2014).2. D. G. Arbó, C. Lemell, S. Nagele, N. Camus, L. Fechner, A. Krupp, T. Pfeifer, S. D. López, R. Moshammer and J. Burgdörfer Phys.Rev. A 92, 023402 (2015).3. X.-M. Tong and S.-I. Chu, Chemical Physics, 217, 119 (1997); X.-M. Tong and S.-I. Chu, Phys. Rev. A 61, 031401 (2000).4. R. Pazourek, S. Nagele, and J. Burgdörfer, Rev. of Mod. Phys. 87, 765, (2015).